Contact protection for mercury wetted switch contacts



E. T. BURTON v2,459,306 CONTACT PROTECTION FOR MERCURY WETTED SWITCBQONTACTS Filed July 21, 1944 Jan. 18, 1949.

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CONDENSER DISCHARGE WITHOUT PROT'EC TING INDuc m/vcs \T/ME DURING WHICH,THREAD OF MERCURY IS OF COMP/4R4 T/VELY HIGH RES/STANCE Patented Jan.18, 1949 UNITED STATES PATENT OFFICE CONTACT PROTECTION FOR MERCURYWETTED SWITCH CONTACTS Everett T. Burton, Millburn, N. J assignor toBell Telephone Laboratories,

Incorporated, New

2 Claims.

This invention relates to circuit makers and breakers and particularlyto problems of contact protection.

The object of the invention is to provide means to prevent undue erosionof contacts of a par ticular nature. The use of mercury wetted contactsis becoming more general and this has led to considerations which mayapply to the operation of what might be termed dry contacts. The mercurywetted contact has been found to have certain particularly valuablequalities and at the same time certain peculiar operatingcharacteristics. By way of example, such contacts when closed eventhrough extremely light contact pressure present extremely low contactresistance whereby comparatively heavy currents may be carried thereby.On the other hand, when such contacts are separated the process involvesthe drawin out of a thread of mercury so that over a given period thecontact resistance changes from a very low resistance throughintermediate values to a comparatively high resistance before thecircuit controlled thereby is definitely broken. Therefore if thecurrent flowing therethrough is comparatively high a considerableheating efiect measured by the FR loss may be developed and if the timeelement is suflicient may produce an explosive effect which will resultin disruption of the mercury surface and exposure of the underlyingcontact metal and the consequent production thereon of a conditioninhibiting the free flow of mercury thereover to repair the damage.

This same efiect may be present on closure of the contacts. Due tosurface tension the mercury presents substantially spherical surfacesapproaching each other and first contact is at a point. There after aneck of rapidly expanding cross-section forms between the two surfaces.Growing under force of surface tension, this neck draws its mercury fromthe two contacts. In the end the neck loses its identity as it mergeswith the mercury of the contacts, resulting in a connection of largecross-section and very low resistance. During a contacting operation,the contact resistance varies with the neck dimensions. Since the periodduring which a neck of small cross-section exists is known to be veryshort, it may be assumed that practically no transfer of heat away fromthe neck occurs during the critical period. Therefore it follows thatsubstantially all the heating due to current through the neck is appliedto the mercury of the neck. It is evident that the safe limit of heatingis reached when the temperature of the mercury neck reaches thevaporizin point. After that, any additional heating results invaporization starting at the center of the neck.

When vaporization occurs, the metallic neck is severed. If the voltageis sufficient, a discharge across the gap is then established resultingin increased voltage drop at the contact with increased heatingsubstantially in direct proportion to the increase of voltage. If thecurrent is momentarily high, for example, at a contact closure, this maybe accompanied by vaporization of explosive violence which drives themercury from the contacts and may even expose the underlying solidcontact surfaces.

It is therefore apparent that to avoid contacting trouble, currents andvoltages must be controlled that the merury necks, on both closing andopening, are not heated to vaporization. Therefore provision must bemade so that on contact closure the initial surge of current, resultingmainly from discharge of capacitance, including both a condenser bridgedacross the contacts and conductor capacitances which under somecondition may have important values, must be regulated to not exceed thesafe thermal storage capacity of the neck at any instant during thefirst few microseconds. At contact opening the protection must preventrapid voltage rise during the drawing of the mercury neck so thatvaporization does not occur. Furthermore, the voltage between theopening contacts during the initial stage of retraction of the ends ofthe broken mercury thread must not be permitted to reach the arcingpotential.

Some authorities hold that any contacting surface carrying current eventhough solid metal becomes momentarily fluid at the instants of eitherfirst closure or final contact before opening. The present inventiontherefore is intended to provide contact protection for all fluidcontact devices whether the contacts are originally fluid or becomefluid only for a short time, the protection being provided to avoid ruinof the contact through the explosive effect of a too high currentconcentration.

In accordance with the present invention, circuit elements are employedto limit the flow of current through the comparatively highresistancc'mercury neck both on closing and opening operations.Generally speaking such elements consist of a capacitance effective onopening of the contacts and an inductance effective on the closure ofthe contacts. In its simplest form the protective network consists of aseries impedance leading to a divided circuit, one branch of which leadsthrough a capacitance connected to one of a pair of contacts and theother branch of which leads through an inductance to the other of saidpair of contacts. Other more complicated networks may be employed inaccordance with the functions of the contacts but each will basicallyinclude the simple circuit above described.

A feature of the invention is an electrical contact protecting networkcomprising a series current limiting element and a parallel circuit onebranch of which consists of a capacitance and the other branch of whichconsists of an inductance and the said contacts in series.

Another feature of the invention is an electrical network for protectingcontacts which on closure produce a progressively comparatively high tovery low resistance circuit consisting of a circuit element having timincharacteristics to prevent a great flow of current durin thecomparatively high resistance period of said contact closure.

The drawings consist of a single sheet having five figures as follows:

Fig. 1 is a cross-sectional view of a relay structure employing thefeatures of the present invention;

Fig. 2 is a diagrammatic illustration of the action of mercury wettedcontacts. This figure consists of two lines of diagrams, the firstshowing in three figures the action of the mercury as the contacts closand the second lower line consisting of four small figures showing theaction of the mercury as the contacts move away from each other;

Fig. 3A is a schematic circuit diagram showing the arrangement of theelectrical elements in the circuit used in protecting the contactsurfaces;

Fig. 3B is a similar schematic circuit diagram showing a complexarrangement wherein a pair of moving armatures each having a front andback contact is employed; and

Fig. 4 is a diagram showing the relation between current and time undertwo different conditions, one graph being used to show the action undereach condition.

The relay of the present invention consists of a glass sealed unitlocated in the center of a coil. This glass sealed unit has a movingarmature supported on a reed and arranged to move between a back and afront contact. These contacts are wetted with mercury and the'whole isimmersed in an atmosphere of hydrogen under high pressure.

The relay is built by using a plastic base I with a number of contactpins 2 and 3 arranged in a manner similar to the well-known electronictubes. Secured to the base and enclosing the whole is a metal cover 4.Within this cover and held snugly in position by resilient means, suchas sponge rubber 5, is a coil 6. Spaced axially within this coil is thecontact unit consisting of the glass wall I having a single electrode 8sealed in the lower press 9 and a pair of electrodes l and II sealedthrough the upper press l2.

The lower electrode 8 is in the form of a tube which is useful forevacuating the vessel and later introducing therein a specified amountof mercury and the necessary amount of hydrogen gas under pressure.After this tabulation process, the lower end of the tubular electrode 8is pressed together and sealed by a welding process. In this weldingprocess a small piece of wire I3 is used as one of the electrodes andthis becomes welded to the crushed end of the tube 8 and is allowed toremain. This piece of metal I3 is useful in producing the weld since itscontact With the crushed end of the tube 9 is through an area of smalldimension and therefore the concentration of current is high.

Within the envelope a spring I4 is welded to the upper end of tube 8 andto the upper end of spring l4 there is in turn welded a pair of wires l5laid close togetherand having a surface which is easily wetted bymercury, This comprises a device known as a mercury wick up whichmercury will readily ascend to form a mercury contact surface at theupper extremity thereof. An armature [6 of magnetizable material iswelded to the mercury wick l5 and extends downwardly into a recessformed between the spring I4 and a portion of the tube 8. Within thisrecess a pool of mercury will be held and this will be useful in feedingthe mercury wick l5.

Both the electrode in and the electrode II have attached thereto adownwardly extending contact piece I! and I8 respectively, to whichsmall pieces of contact material l9 and '20 respectively are welded.These materials are wettable by mercury and therefore will maintain aconstantly wetted surface.

Wires 2|, 22, 23, 24 and Y25 will connect the various portions of thedevice to the pins, such as 2 and 3.

This relay is a transfer arrangement comprising mercury wetted contactssealed in a hydrogen atmosphere at 250 pounds pressure. An electrode ofthis type of relay when not in contact presents a smooth rounded surfacewhich for this discussion will be considered a spherical section. Whenin the act of closing, the two spherical surfaces of mercury connectfirst at a point. This is illustrated in Fig. 2, sketches A, B and C. Insketch A the two rounded surfaces are just making the first contact witheach other. Thereafter, as illustrated in sketches B and C, a neck ofrapidly expanding cross-section forms between the two surfaces. Growingunder force of surface tension, this neck draws its mercury from the twocontacts. In the end the neck loses its identity as it merges with themercury of the contacts, resulting in a connection of large crosssectionand very low resistance.

On opening, the two contact surfaces are connected by a neck of reducingcrosssection which draws to a relatively long and fine thread beforebreaking. The fact that a mercury thread extends momentarily between theopening electrodes results in this relay performing a closed transfer;that is, for a few tenths of a millisecond during each switchingoperation the three electrodes are in common electrical contact. This isknown as bridging. Sketches D, E, F and G represent various successivestages in the formation and final breaking of the thread of mercuryextending between the two contact surfaces as such surfaces move awayone from the other.

Contact protection on this type of relay resolvesinto three categories.These may be associated with three transient phenomena, contact closure,bridging and contact opening. The

first and third types are normally of very short duration, in the orderof a few microseconds at most. The second type is usually of aboutthreetenths of a millisecond duration. These three transients occur oncefor each half cycle of relay operation in a connected sequence; theorder being as given above. Since at all other times during a cycle thecontacts are either definitely closed or open, and therefore incapableof more than very, slight power dissipation, it follows that adequateprotection demands effective functioning of the protective networks forthe very short switching time intervals only. It is important to notethat, given adequate protection for the critical transient periods, thecontacts are capable of controlling relatively large powers.

Before entering into discussion of protective networks, it may bedesirable to consider further the phenomena associated with theexistence of the transient mercury necks. During a contacting operationthe contact resistance varies with the neck dimensions. Since the periodduring which a neck of small cross-section exists is known to be veryshort, it is safe to assume that practically no transfer of heat awayfrom the neck occurs during the critical period. Therefore it followsthat substantially all the heating due to current through the neck isapplied to the mercury-of the neck. It is evident that the safe limit ofheating is reached when the temperature of the mercury neck reaches thevaporizing point. After that any additional heating results invaporization presumably starting at the center of the neck.

Where vaporization occurs the metallic neck is severed. If the voltageis sufficient, a discharge across the gap is then established resultingin increased voltage drop at the contact with increased heatingsubstantially in direct proportion to the increase of voltage. If thecurrent is momentarily high, for example, at a contact closure, this maybe accompanied by vaporization ofexplosive violence which drives themercury from the contacts and may even expose the underlying solidcontact surfaces. It is apparent that to avoid contacting trouble,currents and voltages must be so controlled that the mercury necks onboth closing and opening are not heated to vaporization. Experiment hasshown that these necks in their thinnest stage may have considerableresistance in the order of perhaps a few ohms, but that the highresistance stage is short, usually well under a microsecond.

Therefore it becomes apparent that on contact closure the initial surgeof current resulting mainly from discharge of capacitance, must beregulated to not exceed the safe thermal storage capacity of the neck atany instant during the first few microseconds. At contact opening theprotection must prevent rapid voltage rise during the drawing of themercury neck so' that vaporization does not occur. Furthermore, thevoltage between the opening contacts during the initial stage ofretraction of the ends of the broken mercury thread must not bepermitted to reach the arcing potential.

While it is known that electrostatic attraction may considerably affectsurface contours when a voltage difference exists between very slightlyseparated electrodesno attempt has been made to determine this effect onmercury contacts. It is probable that the formation of mercury necks isappreciably altered when relatively high voltages are used.

A network to protect the contacts under the transient conditionsmentioned above is shown in Fig. 3A. The contact consists of a movablearmature l6 moving between a back contact 20 and a front contact IS.

The armature I 6 is employed to connect a load 29 alternately to abattery 30 of voltage E and to ground. To prevent short-circuiting thesource of voltage during the bridging operation, a series impedance Z1is used. In cases where low power is being dealt with, a resistor whosevalue is equal to about the load impedance may be adequate. Where thepower is high, an inductance whose reactance bears that relationship tothe load impedance may be satisfactory. The important feature to beconsidered in each case is that this impedance must be sufficientlylarge to prevent establishment of excessive magnitudes of current duringthe bridging operation. Tentatively l0 amperes has been specified as themaximum current which may be safely handled by the contacts of a relayof this type and dimension. On the other hand, since the load currentmust be supplied through this impedance, there is a practical upperlimit to the magnitude of Z1.

The foregoing discussion of Z1 pertains to normal bridging times in theorder of a few tenths of a millisecond. If the relay is to be used inlocations where severe jolting or tipping may occur, the value of Z1must be adequate to prevent damaging magnitudes of current in case thefree mercury by such rough treatment is thrown into the contactassembly. It is apparent that where the supply is direct current, Z1must include series resistance for current limitation unless some formof circuit interrupter is employed. Where alternating current is usedthe resistance is not necessary, providing Z1 comprises adequateinductance.

Where Z1 represents load impedance and E is the RMS supply voltage, thevalue of Z1 may be defined thus:

ohms However, in any case Z1 should not be less than Where I0 is themaximum current to be interrupted I I i C nncrofarads (2) This condenservalue is the minimum which may be tolerated. Under some conditions alarger value is to be preferred.

At contact closure it is necessary that the condenser 3| as well as thecapacitance of lead Wires be safely discharged under maximum voltageconditions. Since the mercury neck on initial contact is capable ofsafely absorbing only a small amount of energy, it is required that thedischarge current rise gradually from a low value. The desired result isaccomplished by the use of an inductance 32, preferably of low Q,located at the contact as shown in Fig. 3A. Experimentally it has beenfound that the value of Lrz should be such that the initial rate ofcurrent rise does not exceed 25 amperes per microsecond. Then di/dt=25 X10.

Taking E as the maximum voltage which may occur on the condenser 3|,

This is the minimum value which may be safely used. In certain cases ahigher value may be advantageous.

Consideration of the functioning of the three elements Z1, shown asinductance 33, C1 shown as condenser 3|, and L2 shown as inductance 32,in combination, shows that the interactions which occur in two of thethree combinations are not detrimental. The third combination involvesthe inductance 32 at a contact opening. In this situation L2 is locatednext to the contact and beyond the protective influence of the condenser3|. If prior to contact opening L2 was carrying a current I0, the energystored in its magnetic field L di/dt=E or L microhenries was L2Io /2.The only outlet for this energy on opening of the contact is indischarge across the opening gap. It has been found that this dischargeis relatively innocuous and may be tolerated providing the inductivelystored energy does not exceed about 40 10- joule. A maximum value ofinductance is thus established.

L max=g microhenries (4) Where L2 does not exceed this value, opening ofthe contact results in a weak glow discharge of rapidly dwindlingcurrent at about 30 volts. Its duration is usually less than onemicrosecond.

Upon the closure of contacts [6 and 20 the condenser 3| will dischargethrough the neck of mercury formed, as shown in sketches A, B and C ofFig. 2. Fig. 4 is a graphic illustration of the effect of the inductance32. Without this inductance the current rise, particularly for theinitial period, is very high as shown by graph 34. With the inductance32 in the circuit, however, this characteristic condenser dischargecurve is modified in accordance with the graph 35 so that it will bereadily apparent that the inductance 32 will hold the current value downto a reasonable value during the period when the mercury neck is small.

When contacts of more than one relay are functioning in a system, it ispossible that switching surges of a given contact under certainconditions may reach a contact of another relay during a criticalperiod, thus subjecting the latter to severe transitory conditions. Toavoid this difficulty, it has been found desirable in a circuitutilizing more than one relay to isolate the contacts from each other bymeans of low Q choke coils. This arrangement is illustrated in Fig. 3Bwhere a typical contact system is shown. While the value of these coilsdoes not appear to be at all critical, it is evident that the value ofL3, coil 36 by way of example, must be large with respect to L2,inductance 31 by way of example, and may be small with regard to L1, theinductance value of Z1, element 38. A value per winding of L3 whichappears to be satisfactory is L /L L Coupling between the windings ofthe pairs of La may be desirable in some cases. For example,

the coupling between the L3 windings indicated in Fig. 33 has been foundto be satisfactory.

Isolation of contact electrodes for the band of prominent surgefrequencies by use of L3 coils accomplishes a two-fold result: (a) acontact is permitted to cope with its own transient conditions withoutdetrimental effects of surges introduced from other circuits, and (b)localization of the transient surges to the immediate vicinity of theirorigin may reduce interference in associated apparatus.

In some cases it may be desirable to utilize resistance 39 (R3) and acapacitance 40 (C3) connected in series, forming a shunt across the loadterminals as shown dotted in Fig. 3B. In absence of Le coils this shuntmay be of value in reducing sparking at the relay contacts when the loadis inductive. The value of the resistance may be in the order of theload resistance component or lower. The condenser value should be suchthat the impedance presented to the relay circuit is low for theimportant switching surge frequencies, but the value should not belarger than C1 in any case. Where the L3 coils are used, the importanceof this shunt as a protective device is greatly reduced, although thecombination may be' useful as a filter section for suppressing thecontact surges at the load circuit terminals.

It will thus be seen that contact protection for that type ofcontactwhere a momentary connection of comparatively high resistance ismade on both the opening and the closing'of the contacts andparticularly where this comparatively high resistance connection is inthe form of a thread of mercury, may be provided by a condenser bridgedabout the contacts and an inductance in series therewith. The inductancewill operate on closing of the contacts to limit the high rate ofdischarge of the condenser and the condenser will operate on opening ofthe contacts to dissipate the energy inductively stored in theinductance coil.

What is claimed is:

1. In combination, contacts for making and breaking an electricalcircuit, a pool of mercury, means leading from said pool of mercury toone of said contacts for supplying mercury to said contacts, thesurfaces of said contacts being formed to retain mercury thereon, meansfor moving said contacts toward and away from each other whereby theresistance between said contacts while in contact with each other isextremely low due to the thickness of the column of mercury formedtherebetween but which varies through a wide range to a comparativelyhigh value during the movement apart of said contacts due to the drawingout of a fine thread of mercury before a complete break in contactsbetween said contacts, and an electrical network providing means fortransiently limiting the flowof current through said contacts during thesaid transient high resistance periods of the operation of saidcontacts, said network including an inductance element in series withsaid contacts, and a capacitor bridged about the said series combinationof contacts and inductance element.

2. In combination, contacts for making and breaking an electricalcircuit, a pool of mercury, means leading from said pool of mercury toone of said contacts for supplying mercury to said contacts, thesurfaces of said contacts being formed to retain mercury thereon, meansfor moving said contacts toward and away from each other whereby theresistance between said contacts while in contact with each other isextremely low due to the thickness of the column of mercury formedtherebetween but which varies through a wide range to a comparativelyhigh value during 1he movement apart of said contacts due to the drawingout of a fine thread of mercury before a complete break in contactbetween said contacts and which varies from a comparatively high valueto said extremely low value during the movement together of saidcontacts due to the substantially spherical contour of the mercury onsaid contacts before contact is made therebetween and the fact that thefirst contact between two such spherical surfaces is a point contact andan electrical network providin means for transiently limiting the flowof current through said contacts during the said transient highresistance periods of the operation of said contacts, said networkincluding an inductance element of substantially zero resistance inseries with said contacts and a capacitor bridged about said seriescombination of contacts and inductance element.

EVERETT T. BURTON.

REFERENCES CITED The following references are of record in the file ofthis patent:

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